1. Field of the Invention
The present invention relates generally to liquid coolant for two-phase liquid cooling systems and more specifically, it relates to an azeotrope spray cooling system that utilizes an azeotrope coolant for improving the performance of a spray cooling system.
2. Description of the Related Art
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
Modern electronic devices (e.g. microprocessors, circuit boards, power supplies, and other electronic devices) have increased thermal management requirements. Conventional dry thermal management technology (e.g. fans, vents) simply is not capable of efficiently cooling modern high-end electronics.
Spray cooling technology is being adopted today as the most efficient option for thermally managing electronic systems. U.S. Pat. No. 5,220,804 entitled High Heat Flux Evaporative Spray Cooling to Tilton et al. describes the earlier versions of spray technology, as applied to electronics cooling. U.S. Pat. No. 6,108,201 entitled Fluid Control Apparatus and Method for Spray Cooling to Tilton et al. also describes the usage of spray technology to cool components on a printed circuit board. Spray cooling utilizes phase changing for thermally managing one or more electronic devices. Most spray cooling systems utilize a single component coolant. Spray cooling as used herein, may be a spot spray cooling system as described by '804 to Tilton, wherein the coolant is directed towards modules thermally attached to heat producing devices; or may be a global spray cooling system such as described us U.S. Pat. No. 5,880,931 wherein the cooling fluid is directly applied to the electronics to be cooled.
Spray cooling provides the possibility of removing high heat fluxes with electronic safe fluids that are not optimally suited for use in thermal management systems. The coolant typically used within a spray cooling system is a dielectric fluid (e.g. hydrofluorethers) having a low vaporization temperature at standard atmospheric pressure. One common brand of dielectric coolant for two-phase thermal management systems is manufactured by Minnesota Mining and Manufacturing Company (3M®) under the federally registered trademark Fluorinert®. Although a dielectric fluid is required for use in global cooling systems, it is also suitable for spot cooling systems, as the fluid does not create the risk of system failures, as is the case with water utilizing systems. While Fluorinert is useful for the thermal management of electronic devices, it unfortunately has some relatively poor fluid properties for this use. For example, the thermal conductivity value (0.057 Wm−1K−1) at standard atmospheric conditions limits its ability to conduct heat from a cooling surface of a heat-producing device. For comparison, air has a thermal conductivity value of 0.0267 Wm−1K−1 and water, being much more thermally conductive, has a typical value of 0.611 Wm−1K−1.
More recently, liquid thermal management systems have utilized a mixture of two different components, such as different varieties of Fluorinert, wherein the first component evaporates at a first temperature and a second component may or may not evaporate at a second temperature, wherein the first temperature and the second temperature are different from one another, often referred to as zeotropes. It is important to note that blends of the varieties of Fluorinert are not an azeotropic mixture since the components boil at different points.
An azeotrope is a fluid mixture of two or more components that change phase at nearly the same pressure and temperature. In addition, an azeotrope maintains the same composition as it is boiled (i.e. the vapor has the same composition as the liquid). The components of an azeotrope therefore cannot be separated through simple distillation as with most liquid mixtures, particularly mixtures within the Fluorinert family of fluids. Typically, the component comprising the bulk of the azeotropic mixture is referred to as the base component. Azeotropes are a common byproduct when distilling ethanol, and have been widely used as cleaning fluids, such as a mixture of siloxane and alcohol (e.g. OS-120 brand produced by DOW CORNING CORPORATION).
Some azeotropes are pressure sensitive and some are pressure insensitive, meaning that they are only azeotropes for a limited range of pressures. Azeotropes can be positive or negative; boiling at a temperature above or below that of one of the constituents. The components can be completely miscible or entirely immiscible or anywhere in between. All known immiscible azeotropes are negative boiling.
Several methods exist to separate the components of an azeotrope. One such method is referred to as pressure swing distillation (used for pressure sensitive azeotropes) where the pressure is typically changed to a value away from the point where the activity coefficients of the constituents cross (thereby becoming a zeotropic mixture). Azeotropes are also commonly separated by adding material separating agents (MSAs) to the mixture. This is more commonly used for pressure insensitive azeotropes since for this case pressure swing distillation is ineffective. MSAs often bond to one of the components of the azeotrope to form intermediate azeotropes resulting in the ultimate separation of the initial azeotrope. Constituents can then be removed using common distillation techniques, but ultimately the MSA must be separated. Typically, this is done using filtration.
Though this is not a comprehensive description of all azeotropes or their characteristics, all of these attributes pose potential advantages for use in two-phase liquid cooling systems.